1. Field of the Invention
This invention relates to a control device and control method for an internal combustion engine for correcting fuel injection amount based on the air-fuel ratio. More particularly, this invention relates to a control device and control method for an internal combustion which is operable by an alcohol blended fuel in which fuel is blended with alcohol.
2. Description of the Related Art
A general internal combustion engine, which has an A/F (air-fuel ratio) sensor or O2 (oxygen) sensor disposed in an exhaust pipe, controls an air-fuel ratio based on an output signal from the A/F sensor or O2 sensor. For example, because the A/F sensor is an oxygen sensor having linear output characteristics with respect to the air-fuel ratio, feedback control is executed on fuel injection amount to keep an air-fuel ratio (exhaust gas air-fuel ratio) of exhaust gas at a target air fuel ratio based on the output signal from the A/F sensor.
Also, the internal combustion engine determines the presence of an abnormality in a fuel injection device on the basis of an output signal of the A/F sensor. For instance, when the difference between the target air-fuel ratio that is set depending on an operational state of the internal combustion engine and the exhaust gas air-fuel ratio that is calculated based on the output signal of the A/F sensor becomes greater than a predetermined value that is set beforehand, it is determined that the fuel injection device is not functioning normally.
In recent years, so-called a multifuel internal combustion engine has been used for compensating the disadvantages of a plurality of fuels with different fuel properties and at the same time mutually complementing the advantages of these fuels. A vehicle installed with this multifuel internal combustion engine is generally called “flexible fuel vehicle (FFV).” For example, there is known an FFV which is designed to improve the environmental performance by improving emission performance of the internal combustion engine of the vehicle and discouraging consumption of a fossil fuel such as gasoline fuel by using gasoline fuel, ethanol, or other alcohol fuel alone or a mixture of these alcohol fuels depending on the required performance.
Alcohol fuel, however, is richer in theoretical air-fuel ratio than gasoline fuel because alcohol fuel contains a larger amount of oxygen than gasoline fuel. For this reason, the exhaust gas air-fuel ratio that is fed back by the A/F sensor is shifted to a lean side with respect to the target air-fuel ratio (theoretical air-fuel ratio) of the gasoline fuel, and the fuel injection amount has to be changed to an increase side even when the intake air amount is not changed. As a result, a learning correction value is shifted significantly. Consequently, the difference between the target air-fuel ratio and the exhaust gas air-fuel ratio fed back by the A/F sensor increases excessively, whereby the internal combustion engine might mistakenly determine that the fuel injection device functions abnormally.
For example, a fuel supply device for an internal combustion engine, which is disclosed in Japanese Patent Application Publication No. 03-037343 (JP-A-03-037343), determines the presence of an abnormality in an injector and fuel injection pump based on the intake air amount obtained during idling and the time required for expansion stroke of each cylinder, when the alcohol concentration of fuel and an air-fuel ratio of intake air mixture are within a predetermined range.
The fuel supply device for an internal combustion engine according to the related art described above, however, determines the presence of an abnormality in the injector and fuel injection pump when the alcohol concentration of the fuel and the air-fuel ratio of the intake air mixture are within the predetermined range. Thus, when the alcohol concentration of the fuel and the air-fuel ratio of the intake air mixture are beyond the predetermined range, the presence of an abnormality in the injector and fuel injection pump cannot be determined. Furthermore, because this fuel supply device uses an alcohol sensor to detect the alcohol concentration of the fuel, the structure of the fuel supply device is made complicated and cost increase is incurred.